bug#17569: 24.4.50; doc of "wrapper hooks"

[Drew Adams <drew.adams@oracle.com>]

> > And what about a function, such as `filter-buffer-substring',
> > whose implementation and doc string makes use of such a var?
> > What is being recommended now for someone who wants the
> > functionality of `filter-buffer-substring' - just roll your own?
> 
> Using obsolete variables or functions doesn't automatically make their
> users obsolete.  It just means they need to be fixed by changing their
> implementation to use the alternative non-obsolete replacements.
> Therefore, there's no need to stop using those APIs that use obsolete
> vars.

Yes, we know that.  And you'll notice that I didn't say anything about
the many callers of `filter-buffer-substring' or about code that uses
`filter-buffer-substring-functions'.

However, I see now that I was mistaken about `filter-buffer-substring'
using obsolete variable `filter-buffer-substring-functions'.  It uses
the replacement function `filter-buffer-substring-function' (no `s')
instead.

That was my main concern: what was being recommended as a replacement
for `filter-buffer-substring'.  I see now that it is not being made
obsolete by making `filter-buffer-substring-functions' obsolete.